In recent years, an image display device using a laser (laser display) has attracted attention for its excellent color reproducibility and low power consumption. FIG. 15 shows a schematic construction of a laser display 1200. Light beams emitted from R (red), G (green), and B (blue) laser light sources 1201 to 1203 are transmitted through a speckle removal unit comprising diffusion plates 1205a to 1205c (lenticular lens or rod prism), and intensity-converted by light modulators 1207a to 1207c according to an input video signal, and then multiplexed by a multiplexing prism (dichroic prism) 1208 comprising a dielectric multilayer mirror. Further, a two-dimensional image is displayed on a screen 1210 by a projection lens 1209. In the display thus constituted, since the respective lights from the R, G, B light sources are monochromatic lights, a vivid image having a high chromatic purity can be displayed by using laser light sources of appropriate wavelengths. Further, the light sources can be miniaturized since the laser is used, and the optical system can be miniaturized since focusing of light is facilitated, and thereby a palmtop type image display device can be realized. On the contrary, since the laser having a high coherency is used, interference noise called “speckle noise” undesirably occurs in the display image. Therefore, there have been proposed various methods for removing such speckle noise by generating an optical path difference in a polarization direction using a prism (Patent Document 1), by vibrating optical components such as two-dimensional beam scanning means 1204a to 1204c or diffusion plates 1205a to 1205c as shown in a schematic diagram of FIG. 15 to change beam paths of light sources, thereby making random the wave front of light that irradiates a screen (Patent Document 2), by generating a side band in a spectrum using a light modulator to broaden the apparent spectrum of light (Patent Document 3), by operating an oscillation wavelength using a technique of performing injection seeding into a solid laser (FIG. 16: conventional construction) (Patent Document 4), or by using semiconductor lasers of plural wavelengths which are modularized (Patent Document 5).
Patent Document 1: Japanese Published Patent Application No. 2004-151133
Patent Document 2: Japanese Published Patent Application No. 2004-138669
Patent Document 3: Japanese Published Patent Application No. Hei. 9-121069
Patent Document 4: Japanese Published Patent Application No. Hei. 10-294517
Patent Document 5: Japanese Published Patent Application No. 2004-144794